There are four main types of roofers: shinglers, who primarily install shingles, shakes, tiles, and other nail-on products on roofs with 5:12 pitches or above; metal roofers, who focus on metal panels; single-ply or "flat" roofers, who focus on roofs such as single-ply or foam roofs; and "hot" roofers, who work using tar-based products. It is not uncommon, however, for companies to have their roofers service multiple styles and types of roofing; and certain manufactures will allow only pre-approved installers, thus making these four roofer types limiting. As per the application areas, roofing contractors can be categorized as Industrial or commercial roofers, Factory Shed roofers, Residential Roofers, Commercial roofers. Based on the types of the materials, roofing can be classified as terrace roofing, metal roofing, polycarbonate roofing, steel roofing, PVC roofing, skylight roofing. Pre-engineered companies installs roofing.